The following discussion of the background of the invention is merely provided to aid the reader in understanding the invention and is not admitted to describe or constitute prior art to the present invention.
The activation of antigen presenting cells (APCs) including the dendritic cells (DCs), followed by loading of the antigen presenting cell with relevant antigens, is a requisite step in the generation of a T cell dependent immune response against cancer cells. Once activated and loaded with tumor antigens, DCs migrate to regional lymph nodes (LNs) to present antigens (ags) to T cells. Very commonly, these APCs express insufficient amounts of surface activation molecules (1) which are required for optimal activation and expansion of T cell clones competent to recognize tumor antigens. Antigen (ag) presentation to naive T cells, in the absence of costimulatory molecule expression on the surface of the APC, leads to anergy of T cells (2). Moreover, cross-presentation by DCs without CD4+ T cell help also results in peripheral deletion of Ag-specific cells in regional LNs (3). In contrast, in the presence of CD4+ T cell help, DCs change their functional ability to cross-prime cells, resulting in clonal expansion of effector T cells (4). This CD4+ T cell help can be replaced with CD40-CD40 ligand (CD40L) interactions (5). CD40L is a 33-kDa type II membrane protein and a member of the TNF gene family which is transiently expressed on CD4+ T cells after TCR engagement (6).
The ability of DCs to generate anti-tumor immune responses in vivo has been documented in a number of animal tumour models (7, 8). However, DC-mediated induction of immunity represents a major therapeutic challenge. The current procedures used for isolating and activating DCs are resource intensive and are difficult to apply to routine clinical practice. In addition, it is difficult to ensure that the antigen presenting cells express appropriate adhesion molecules and chemokine receptors to attract the these DCs to secondary lymphoid organs for priming T cells (9-14).